The invention relates to a transparent, biaxially oriented polyester film having a base layer at least 80% by weight of which is composed of a thermoplastic polyester, and having an outer layer which has good mechanical stability. The invention furthermore relates to the use of the film and to a process for its production.
Food and drink packaging frequently requires a high barrier effect with respect to gases, water vapor and flavors. In a process commonly used to produce packaging of this type, the plastic films used for this purpose are metallized with aluminum in a high vacuum. Another commonly used process is to coat the films with ceramic materials, such as SiOx, AIOx or MxOx. The barrier effect with respect to the substances mentioned above is essentially dependent on the type of the polymers in the film and the quality of the barrier layers applied. For example, metallized, biaxially oriented polyester films have a very high barrier effect with respect to gases such as oxygen and flavors. Due to their good barrier properties, metallized or ceramically coated films are used in particular for packaging foodstuffs and other consumable items where long storage times or transport times lead to a danger that, if the barrier is inadequate, the packaged food or drink may spoil, become rancid, or lose its taste, for example in the case of coffee, fat-containing snacks (nuts, potato chips, etc.) or drinks containing carbon dioxide (in pouches). In future, the demands of the packaging industry will move in the direction of yet higher barriers in coated films of this type. On the other hand, there are types of packaging which have to have a low barrier, but should be transparent and cost-effective. Polypropylene films coated with polyvinylidene chloride (PVDC) are used here, for example. Although films coated with PVDC are transparent, the coating, like the metallization, takes place in a second operation, which once again markedly increases the cost of the packaging. Ethylene-vinyl alcohol copolymers (EVOH) likewise exhibit a high barrier effect. However, films modified with EVOH are particularly severely affected by moisture, and this restricts their range of application. In addition, due to their poor mechanical properties they are relatively thick or have to be laminated with other materials at high cost. In addition, they are difficult to dispose of after use. Some raw materials, furthermore are not approved by the authorities or are unsuitable for producing food and drink packaging.
In all of these types of packaging, good mechanical resistance to puncturing or splitting (puncture resistance) is important for protection of the contents.